The present disclosure relates to methods of printing wetness indicators on hygienic articles.
Methods have been proposed to indicate wetness in a hygienic article, namely diapers or incontinence pads. For example, the inner surface (or the surface towards the body) of the outer liner of a diaper may have a printed message such as animal figures, alphabets, or other patterns attractive to children (infants or toddlers). When the diaper becomes wet and the urine reaches the outer liner, the figure or pattern in the printed message will dissolve or otherwise be changed. Such changes in the image will be visible to the wearer or caregiver, thereby indicating that a diaper change is required.
Ink jet printing is a well-known technique by which printing is accomplished without contact between the printing device and the substrate on which the printed characters are deposited. Briefly described, ink jet printing involves projecting a stream of ink droplets to a surface in a controlled manner so that the droplets are caused to form the desired printed image on that surface. This technique of noncontact printing is particularly well suited for the application of characters onto substrates such as diapers. Ink jet printing can be broadly divided into drop-on-demand (DOD) printing, and continuous inkjet (CIJ) printing. In DOD printing, droplets are produced as needed by the movement of the piezo or the generation of a bubble to form an image on the substrate. The latter is known as thermal ink jet (TIJ). In CIJ printing, drops are produced continuously, but only a fraction of the drops are used to form an image. During CIJ printing, the drops are selectively charged and deflected in an electrical field. Printed drops are charged and deflected to form the print image and the non-charged drops are returned to the gutter. The amount of charge determines the relative position of the printed drops on the substrate. In binary array technology, the non-printed drops are charged and deflected to the gutter and the printed drops are not charged. As is the case with DOD technology, the relative positions of the nozzles in the array determine the relative position of the printed drops.
There are several advantages with the binary array technology. In comparison to the single nozzle CIJ technology, binary array printing technology offers better print quality because the alignment of the printed drops does not depend on the ability to accurately charge these drops. In addition, due to the number of nozzle orifices in the array, binary array is particularly advantaged to print large graphics or multiple line images at high line speed.
For the production of hygienic articles, inks are preferably laid down within a single pass of the printhead over the same area of the substrate and printers should be easily incorporated into existing production lines. Digital printers based on DOD Piezo or TIJ technology can not typically apply enough ink volume during a single pass at very high speeds to achieve a high degree of print color contrast. A virtue of binary array printing technology over drop-on-demand ink jet is the delivery of relatively large volume ink jet drop. Multiple drops can also be applied at essentially the same physical spot to promote the printed color and contrast. In addition, piezo DOD inks are typically based on oil or other non-volatile solvents and thus do not dry on non-porous or semi-porous substrates. TIJ inks are typically water based but typically contain a substantial amount of non-volatile solvents, again not suited for printing on non-porous and semi-porous plastic films or non-woven materials. In addition, drop generation rates of CIJ printing are higher than those of either DOD or TIJ printing. As a consequence of all of these factors, the line speed of DOD or TIJ printing is much slower than the CIJ binary array. Conventional contact printing methods such as flexography require a relatively large footprint on a production line and thus are not preferred for reasons of inconvenience and cost of integration.
One type of TIJ-based technology that resembles binary array is Kodak STREAM, which has recently been introduced. Kodak STREAM is a continuously recirculating ink technology, but instead of using a piezo signal to induce drop breakoff, it uses a modulated series of thermal pulses at each of the individual nozzles. Like binary array, STREAM's is theoretically capable of forming printing drops at a rate exceeding 100 thousand per second, so it would be also be suited to very high speed, high quality printing of hygienic articles if suitable fast-drying inks were to be employed.
In general, an ink jet ink composition must meet certain strict requirements to be useful in ink jet printing operations. These requirements relate to the physical properties of the fluid (such as viscosity and resistivity), the chemical properties (such as the solubility and compatibility of the components), and the ability to suitably wet the substrate. Further, the ink must be quick drying and smear resistant, and be capable of passing through the ink jet nozzle(s) without clogging, and permit rapid cleanup of the machine components with minimum effort.
Previous wetness indication ink jet inks have dry times on non-porous substrates greater than 3 seconds, and most are greater than 5 seconds. In addition, such inks are typically not designed for binary array printers.